Such coils, when used as the inductive windings of transformers operating at a medium or high voltage, must withstand considerable voltage differences between adjacent turns. According to the conventional mode of manufacture, the coil is wound in a succession of cylindrical layers each usually consisting of a number of turns considerably exceeding the number of coaxial layers. The term "wire", as here used, may comprise a single conductor (generally of copper) or, possibly, several conductors twisted together or parallel to one another which are wound jointly to form the coil.
With this mode of winding, superimposed turns at the ends of adjacent layers will be electrically separated by considerable lengths of wire so that significant voltage differences may exist therebetween. This frequently necessitates the interposition of sheets of paper or other dielectric material between the layers in order to avoid breakdowns. Even if the usual wire insulation of enamel or the like suffices for normal operation, such breakdowns could occur in the event of overvoltages due, for example, to lightning discharges. To test the breakdown resistance of the insulation, it is common practice to subject the coil to a "shock wave" in the form of a high-voltage pulse with a steep wavefront which will be considerably higher than the nominal operating voltage of the coil or the transformer. Such a pulse gives rise to an exponential distribution of the voltage along the winding, with a steep voltage gradient at the extremities close to its terminals. This distribution is due primarily to the capacitances existing between adjacent turns within the coil body.